Xscope sits in-between an X11 client and an X11 server and prints the
contents of each request, reply, error, or event that is communicated
between them.

Xscope can decode the core X11 protocol and several extensions,
including BIG-REQUESTS, LBX, MIT-SHM, NCD-WinCenterPro, RANDR, and
RENDER.

This information can be useful in debugging and performance tuning of
X11 servers and clients.
